Receiver and a method for mobile communications

ABSTRACT

In a method and a mobile communications receiver for processing signals from a first cell and a second cell a timing of the signal from the first cell and the second cell is obtained. A timing difference (δ) between the timings of signals from the first and the second cell is determined and based on that a timing (κ) for a window for discrete Fourier transform, DFT, processing is adjusted. DFT processing of the signals using the timing (κ) of the DFT window is then performed.

TECHNICAL FIELD

The present invention relates to a method and a receiver for processinga signal in a mobile communications system.

BACKGROUND

Recently, an increased demand for high data rates in mobilecommunications has been seen, and this trend will most likely continuein the coming years.

In order to meet this demand, new transmission techniques have beendeveloped. In the forthcoming evolution of present mobile cellularstandards like GSM and Wideband Code Division Multiple Access (WCDMA),Orthogonal Frequency Multiple Access (OFDM) will be used fortransmission. OFDM promises higher data rates and a more efficient usageof limited bandwidth resources than the presently employed techniques.

Furthermore, in order to have a smooth migration from the existingcellular systems to a new high capacity and high data rate system inexisting radio spectrum, such a new system has to be able to operate ina flexible bandwidth. A proposal for such a new flexible cellular systemis 3G Long Term Evolution (3G LTE) that can be seen as an evolution ofthe 3G WCDMA standard. This system will use OFDM as multiple accesstechnique (called OFDMA) in the downlink and will be able to operate onbandwidths ranging from 1.4 MHz to 20 MHz. Furthermore, data rates up to100 Mb/s will be supported for the largest bandwidth, and such datarates will be possible to reach using MIMO (Multiple-Input-MultipleOutput) schemes in the down-link.

In such a system, and in a situation where a mobile device is surroundedby a number of cells, problems relating to strong inter-cellinterference (ICI) may occur. In order to optimize the throughput alsoin such a situation, the mobile device needs to implement methods forhandling such inter-cell interference.

SUMMARY

A method of processing a signal in a mobile communications system ispresented, wherein the inter-cell interference is mitigated in areceiver by adjusting the position of a window for discrete Fouriertransform, DFT, processing based on a timing difference between a firstcell and a second cell. In this way a possible joint channel estimateprocedure and, in turn, the quality of the demodulated symbols areimproved.

Specifically, according to embodiments of the invention, in a method, ina mobile communications receiver, of processing signals received from afirst cell and a second cell, a timing of the signal from the first cellis obtained, and a timing of the signal from the second cell isobtained. A timing difference between the timings of the signals fromthe first and the second cell is determined and a timing for a windowfor DFT, processing is adjusted based on the timing difference. Thesignals are then DFT processed using the adjusted timing of the DFTwindow.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described more fully below with reference to thedrawings, in which:

FIG. 1 is a schematic view of a receiver arrangement.

FIG. 2 shows part of a mobile communications system.

FIG. 3 illustrates received signals from two cells.

FIG. 4 shows power delay profiles for two received signals.

FIG. 5 illustrates received multipath signals from two cells.

FIGS. 6A and 6B illustrates the effective Signal to Noise Ration (SNR)as a function of DFT window placement for two different timingdifferences.

FIG. 7 shows parts of a receiver according to embodiments of theinvention.

FIG. 8 shows a flow diagram of embodiments of the invention.

DETAILED DESCRIPTION

To facilitate an understanding of exemplifying embodiments of theinvention, many aspects are described in terms of sequences of actionsthat can be performed by elements of a computer system. For example, itwill be recognized that in each of the embodiments, the various actionscan be performed by specialized circuits or circuitry (e.g., discretelogic gates interconnected to perform a specialized function), byprogram instructions being executed by one or more processors, or by acombination of both.

In FIG. 1 a receiver 10 is shown which receives signals via a receiverantenna 12. In the receiver 10, the signals received by the antenna arefirst processed by a radio front end (RF) unit 14, then by a baseband(BB) unit 16 and then possibly by some additional unit 18.

In FIG. 2, a mobile device 20 is connected to a first cell 22 (servingcell) of a radio base station 24 (also called Node B). The mobile devicecomprises a receiver 10 of FIG. 1. The mobile device is surrounded by anumber of neighboring cells 26, associated with the same or another basestation. In each cell, pilot symbols are transmitted to be used bymobile devices when obtaining channel estimates for that cell. However,the transmission of pilots from the neighboring cell(s) also causesinterference when the mobile device 20 is obtaining channel estimatesfor the serving cell 22. To improve the situation, joint channelestimation may be used, where the receiver calculates channel estimatesbased on both the pilot symbols from the serving cell and for one ormore neighboring cells. In this disclosure, only one neighboring cell isused in the exemplifying embodiments, but extension to more than oneneighboring cell is straightforward.

When a receiver calculates channel estimates in an OFDM based system,(or a system employing any access technique with DFT processing in thereceiver, such as single-carrier frequency division multiple access,SC-FDMA) a timing κ for a window for DFT processing needs to be set. InFIG. 3, u(t) denotes the signal from the serving cell and v(t) denotesthe signal from the neighboring cell. CP denotes the cyclic prefix partof the signals. The neighboring cell signal v(t) is delayed δ samples (δmay be negative or positive) compared to the serving cell signal u(t).

It may be noted that v(t) may also be a signal from a second servingcell. This may be the case when COordinated Multipoint transmission(COMP) is used.

A first option is to place the DFT window such that the start of thewindow is somewhere in the cyclic prefix of the serving cell signalu(t). This would be the timing used when obtaining channel estimatesonly for the serving cell.

However, when the mobile device calculates improved channel estimatesusing a joint channel estimation approach with a neighboring cell withtypically other timing than the serving cell—in this case the timingdifference is δ—relying placement of the DFT window solely on theserving cell may lead to unnecessary Inter Signal Interference (ISI),and, hence, lower quality channel estimates.

According to embodiments of the inventions, the timing difference δbetween the serving cell signal u(t) and the neighboring cell signalv(t) is taken into account for determining the timing of the DFT window.This may be done in different ways. One option is to place the DFTwindow so that the start of the window is at δ/2.

Another option is to take the signal power of the serving cell and theneighboring cell into account and use the ratio between the power of theneighboring cell and the serving cell when determining the timing forthe window, according to:

$\kappa = \frac{\delta \cdot P_{NBC}}{P_{SC} + P_{NBC}}$

wherein κ is the timing of the DFT window, P_(NBC) is the signal powerof the neighboring cell, and P_(SC) is the signal power of the servingcell. The signal power of the serving cell and the neighboring cell maye.g. be obtained from Reference Signal Received Power (RSRP)measurements.

A weighting factor α, 0≦α≦1, possibly predetermined in simulations, maybe included according to:

$\kappa = \frac{\delta \cdot \alpha \cdot P_{NBC}}{{\alpha \cdot P_{NBC}} + P_{SC}}$

These two expressions both assume a single tap channel, but it wouldalso be possible to extend the expressions to a multi-tap channel. Inthat case a power delay profile is determined for the signals, such asis illustrated in FIG. 4. The PDP comprises a number of channel taps h₀,h₁, g₀ and g₁. h₀ and h₁ are associated with the signal from the servingcell and g₀ and g₁ are associated with the signal from the neighboringcell. Each channel tap has a respective tap power, which is symbolizedby the height of the taps in FIG. 4.

In FIG. 5 the four different channel taps are illustrated as well, andhere n₀, n₁, n₂ and n₃ are the number of samples in the DFT window thatdo not belong to the symbol that is being estimated.

A timing of the window can also be calculated by minimizing the sum ofthe tap powers for the channels taps h₀, h₁, g₀ and g₁, multiplied bythe respective number of excess samples, n₀, n₁, n₂ and n₃, outside thesymbol that is estimated. In FIG. 5 this would be equal to minimizingthe dotted areas. The width of these areas symbolizes the number ofsamples which do not belong to the symbols that are being estimated. Theheight of each dotted area symbolizes the channel tap power. Thus, inthe illustrated example we want to choose a timing for the window, i.e.a value of κ, that minimizes

n₀·|h₀|²+n₁·|h₁|²+n₂·|g₀|²+n₃·|g₁|²,

wherein the straight brackets means the absolute value.

Another way of expressing this would be by starting with the expressionfor the received signal after the DFT processing. Note that thefollowing discussion, for the sake of simplicity, has been limited to aone tap channel, but, again, extension to a multitap channel isstraightforward.

The signal after DFT can be described by:

y(t)=H(t)u(t)+G(t)v(t)+{tilde over (H)}(t)ũ(t)+{tilde over (G)}(t){tildeover (v)}(t)+n(t)

Here H(t) is the channel for the serving cell and G(t) the channel forthe neighboring cell. {tilde over (H)}(t)ũ(t) and {tilde over(G)}(t){tilde over (v)}(t) model the ICI and the ISI, i.e. these twoterms correspond to the samples which are inside the DFT window, butoutside the symbol that is being estimated, this in turn correspondingto what is shown as the dotted areas in FIG. 5. n(t) is the backgroundnoise.

For the serving cell signal u(t) the number of samples N_(u) beingoutside the symbol of interest but still in the DFT window is:

$N_{u} = \left\{ \begin{matrix}{{- \kappa} - N_{CP} - 1} & {\kappa < {{- N_{CP}} - 1}} \\0 & {{{{\kappa>={{- N_{CP}} - 1}}\&}\mspace{11mu} \kappa}<=0} \\\kappa & {\kappa > 0}\end{matrix} \right.$

For the neighboring cell signal v the number of samples N_(v) that areoutside the symbol of interest but still in the DFT window is:

$N_{v} = \left\{ \begin{matrix}{{- \kappa} + \delta - N_{CP} - 1} & {{\kappa - \delta} < {{- N_{CP}} - 1}} \\0 & {{{{{{\kappa - \delta}>={- N_{CP}}}\&}\mspace{11mu} \kappa} - \delta}<=0} \\{\kappa - \delta} & {{\kappa - \delta} > 0}\end{matrix} \right.$

In these expressions, N_(cp) is the number of samples in the cyclicprefix of the symbol. Finally, the variance of the disturbance e_(tot)of the joint channel estimate due to ICI, ISI and background noise canbe calculated as

${V\left\lbrack e_{tot} \right\rbrack} = {{{{V\left( {{H(t)}{u(t)}} \right)} \cdot 2}\frac{N_{u}}{N_{DFT}}} + {{{V\left( {{G(t)}{v(t)}} \right)} \cdot 2}\frac{N_{v}}{N_{DFT}}} + \sigma^{2}}$

Here N_(DFT) is the number of samples in the DFT window, or in otherwords, the length of the DFT window. The variance of the backgroundnoise is σ². By minimizing this expression a timing κ for the DFT windowcan be found.

The effective SNR is equal to the signal power V(H(t)u(t)) divided bythe variance of the disturbance e_(tot). In FIGS. 6A and 6B theeffective SNR is shown for different timings κ of the window for DFTprocessing. FIG. 6A shows the case where the timing difference δ betweenthe serving cell and the neighboring cell is smaller than the cyclicprefix and FIG. 6B shows the case where δ is larger than the cyclicprefix. By using superposition it is possible to also calculate aposition for the DFT window for other types of dispersive channels.

An alternative way of describing this is that the variance of thechannel estimate is minimized for κ.

As a further option for determining how the timing of the DFT windowshould be adjusted, the variance of the error in the demodulated signal,can be minimized for κ.

The demodulated signal is equal to the DFT processed signal y(t) dividedby an estimate of the channel received from a (joint) channel estimationunit. Since both y(t) and the channel estimate depend on κ and δ, thevariance of the demodulated signal will also be dependent on κ and δ. Avalue of κ may be then be found by minimizing the variance of thedemodulated signal for κ.

In FIG. 7 relevant parts of the baseband unit 16 of the receiver 10 areshown. A cell search unit 28 detects the neighboring cell and determinesthe timing and the power delay profile of the serving cell and theneighboring cell from the received signals. A control unit 30 determinesthe timing difference δ based on the timings of the serving cell and theneighboring cell, and then determines an adjustment of the timing κ forthe window for DFT processing based on at least the timing difference δaccording to any of the methods discussed above. A DFT unit 32 removesthe cyclic prefix and places the window for DFT processing according tothe determined timing κ and then DFT processes the received signal. Inthis context, it may be noted that the DFT unit often is implemented asa unit performing fast Fourier transform (FFT) processing.

A joint channel estimation unit 34 may then calculate a joint channelestimate using the DFT processed signal and information regarding pilotsymbols and their frequency location conveyed through the cell id forthe serving cell and the neighboring cell which is received from thecell search unit 28. A demodulation unit 36 then demodulates the signal.

In some embodiments of the invention, the joint channel estimation isonly performed in case the control unit determines that the ratio of thesignal power of the neighboring cell to the serving cell is above apredetermined threshold, e.g. if the neighboring cell is less than 10 dBweaker than the serving cell.

Additionally, the time alignment between the serving cell and theneighboring cell, i.e. δ, may be used to determine if joint channelestimation is to be performed. E.g. if δ is larger than 3-5 times thelength of the cyclic prefix, channel estimation may be based only on theserving cell.

In FIG. 8, a method according to embodiments of the invention isillustrated. In step 38, a timing is obtained for a first and a secondcell. In the above discussed example, the first cell is a serving celland the second cell is a neighboring cell. In step 40 the timing κ ofthe window for DFT processing is determined based at least on the timingdifference δ between the timings of the first cell and the second cell.In step 42 DFT processing is performed using the determined timing κ ofthe DFT window.

It may be noted that the above discussion is related to a singletransmitter/single receiver antenna case, but extension to severaltransmitter and/or receiver antennas would be straightforward.

Although reference is here made to a receiver in a mobile device, suchas a mobile terminal or a user equipment (UE), it should be noted thatthe methods and apparatus described may be used at anytelecommunications receiver, i.e. in a mobile station or a base station,and the transmission may be uplink or downlink.

Thus, the embodiments disclosed herein are merely illustrative andshould not be considered restrictive in any way. The scope of theinvention is given by the appended claims, rather than the precedingdescription, and all variations which fall within the range of theclaims are intended to be embraced therein.

1. A method, in a mobile communications receiver, of processing signalsfrom a first cell and a second cell, the method comprising: obtaining atiming of the signal from the first cell; obtaining a timing of thesignal from the second cell; determining a timing difference between thetimings of signals from the first and the second cell; adjusting atiming for a window for discrete Fourier transform, DFT, processingbased on the timing difference; performing DFT processing of the signalsusing the timing of the DFT window; obtaining a power delay profile ofthe first cell and a power delay profile of the second cell, the powerdelay profiles comprising a number of channel taps having respective tappowers; and adjusting the timing of the window by minimizing the sum,over all channel taps, of the respective tap power multiplied with arespective number of samples that are covered by the FFT window but areoutside a symbol of interest.
 2. (canceled)
 3. (canceled)
 4. The methodof claim 1, further comprising determining a joint channel estimateassociated with the first and the second cell.
 5. The method of claim 4,further comprising: determining a joint channel estimate associated withthe first and the second cell, only if a ratio between a signal power ofthe second cell and a signal power of the first cell is above apredetermined signal power threshold.
 6. The method of claim 4, furthercomprising: determining a joint channel estimate associated with thefirst and the second cell only if the timing difference is below apredetermined timing threshold.
 7. The method of claim 4, furthercomprising: adjusting the timing of the window so that a variance of thejoint channel estimate is minimized.
 8. The method of claim 1, furthercomprising: demodulating the DFT processed signal to produce ademodulated signal; and adjusting the timing of the window so that avariance of an error of the demodulated signal is minimized.
 9. A mobilecommunications receiver, comprising: a cell search unit configured toobtain a timing of a signal from a first cell and a timing of signalfrom a second cell; a control unit configured to determine a timingdifference between the timing of the signal from the first cell and thetiming of the signal from the second cell and to adjust a timing for awindow for discrete Fourier transform, DFT, processing based on thetiming difference; and an FFT unit adapted to DFT process the signalsusing the timing of the DFT window, wherein the cell search is furtheradapted to obtain a power delay profile of the first cell and a powerdelay profile of the second cell, the power delay profiles comprising anumber of channel taps having respective tap powers, and the controlunit is further adapted to adjust the timing of the window by minimizingthe sum, over all channel taps, of the respective tap power multipliedwith a respective number of samples that are covered by the FFT windowbut are outside a symbol being estimated.
 10. (canceled)
 11. (canceled)12. The receiver of claim 9, further comprising a joint channelestimation unit configured to determine a joint channel estimateassociated with the first and the second cell.
 13. The receiver of claim12, wherein the joint channel estimation unit is configured to determinea joint channel estimate associated with the first and the second cell,only if a ratio between a signal power of the second cell and a signalpower of the first cell is above a predetermined signal power threshold.14. The receiver of claim 12, wherein the joint channel estimation unitis configured to determine a joint channel estimate associated with thefirst and the second cell, respectively, only if the timing differenceis below a predetermined timing threshold.
 15. The method of claim 12,wherein the control unit is adapted to adjust the timing of the windowso that a variance of the joint channel estimate is minimized.
 16. Themethod of claim 9, wherein the control unit is adapted to adjust thetiming of the window so that a variance of an error of the demodulatedsignal is minimized.